Modern magnetic disk drives require a larger capacity, higher recording density, and a faster access speed than before, and are typically equipped with a fast rotating disk and a fast moving head-gimbal assembly. Fast rotation and movement bring about air turbulence, causing vibration to the disk and head-gimbal assembly. Air turbulence and vibration prevent the head from being positioned accurately and rapidly on the disk having densely recorded data thereon. Incapability of predicting the magnitude and cycle of randomly occurring air turbulence makes the desired positioning more complex and difficult. Moreover, the magnetic disk drive with air turbulence and vibration makes undesirable noise.
Among other problems arising from air turbulence due to rapid rotation in the magnetic disk drive is an increased power consumption. The rapidly rotating disk drags its neighboring air, generating a shear force at the boundary between the dragged air near the rotating disk and the stationary air away from the rotating disk. This shear force is a load that opposes the disk rotation, and hence it is called windage loss. The greater the rotating speed, the larger the windage loss. The motor running at a high speed against this windage loss needs a large electric power.
Noting the fact that the air turbulence and windage loss increase in proportion to the density of gas in the magnetic disk drive, there has been proposed an idea of reducing air turbulence and windage loss by replacing air in the sealed magnetic disk drive with a gas lighter than air.
Candidates for lighter-than-air gas include hydrogen and helium, with the latter being more desirable from the practical point of view. The sealed magnetic disk drive filled with helium gas is free of the above-mentioned problems and hence is capable of rapid and accurate positioning control, with quiet operation and power saving.
Unfortunately, helium gas has such a small molecule size and such a large diffusion coefficient that during operation it easily leaks out from the ordinary magnetic disk drive whose enclosure lacks complete airtightness.
U.S. Patent Publication No. 2005/0068666 (“Patent Document 1”) discloses an airtight structure capable of retaining an easy-to-leak low-density gas such as helium.
According to Patent Document 1, the magnetic disk drive has an enclosure constructed as shown in FIG. 7A (which is a sectional view). The enclosure 100 is comprised of a base 120, a side wall extending from the base 120, and a cover 110 laser-welded to the top of the side wall. The interior 102 of the enclosure houses the HDD unit 101. The step of attaching the cover 110 is accomplished in an atmosphere of helium gas so that the interior 102 of the enclosure is filled with helium gas. With the cover 110 hermetically sealed, the resulting magnetic disk drive contains helium gas filling the interior 102 of the enclosure.
Patent Document 1 also discloses another airtight structure for the magnetic disk drive as shown in FIG. 7B (which is an enlarged sectional view). This airtight structure of dual cover type consists of an inner cover 240 and an outer cover 110. The inner cover 240 rests on the flange 221 of the base 120, with their gap completely filled with a non-airtight seal 242 inserted therein. The outer cover 110 is welded to the top of the side wall of the base 120.
The airtight structure mentioned above is not complete because the magnetic disk drive has to be mounted on an external unit by means of tapped holes made in its base according to its standard. So long as the base is formed by die-casing, such tapped holes tend to permit a low-density gas to leak out from the interior of the magnetic disk drive even though the base is hermetically closed by the cover.
Gas leakage through tapped holes is due to the nature of die casting. Molten aluminum alloy cast into a mold cools and solidifies inward, but not evenly throughout. However, it easily fills uniformly large parts in the mold cavity and then it cools and solidifies therein at a constant rate, thereby allowing the resulting die-cast product to have a uniform density.
However, the foregoing does not hold true for those parts of the mold in which tapped holes are made. Such parts have thicker and more complex shapes than their surrounding parts and hence prevent the melt from uniform flow, complete filling, and rapid cooling, but allow it (remaining hot longer than its surroundings) to expand during cooling. The resulting die-cast product has many shrinkage cavities in which the aluminum alloy has a decreased density. These shrinkage cavities lend themselves to passage of the low-density gas from the interior of the magnetic disk drive to the tapped holes.
The magnetic disk drive having an enclosure filled with a low-density gas is subject to gas leakage mostly through tapped holes made in the bottom of the enclosure. For prevention of gas leakage, it is necessary to reduce shrinkage cavities that occur near tapped holes.